A screw of this type is known from German patent specification 39 26 000. This screw, regarding which the patent specification expressly points out that it is particularly suitable for setting and adjusting devices, that is to say for so-called movement threads, is to have a high-quality thread, by means of which a continuous and exactly meterable screw-in torque can be achieved. The outlay in terms of forming during the production of the screw by cold rolling is, at the same time, to be lower than in the case of conventional types of screw. In order to achieve this object, the screw possesses a vertex angle of about 40.degree., thus making it easier for the screw material to flow during the cold-rolling process.
The present invention is to provide a screw which, in contrast to the action of a movement thread, is particularly suitable for screwing, in particular, into thermoplastics for fastening purposes. When screws of this type are screwed into plastic, the latter is displaced by the thread teeth penetrating into the plastic, for which purpose the thread cross section must make sufficient free space available. During this time, the plastic displaced by the thread teeth is squeezed into this free space. In this case, German patent specification 27 54 870, column 2, paragraph 1, started from the idea of penetrating as deep as possible into the respective plastic workpiece (high bearing depth), for which purpose, in the case of a flank angle of about 30.degree., a correspondingly large free space must then be made available. To this effect, in the screw according to German patent specification 27 54 870, the thread bottom has a contraction, so that a correspondingly large free space is available for the plastic displaced by the thread teeth. In this case, the plastic displaced by the thread teeth has to cover, from the region of the thread tooth which is penetrated into the plastic, into the thread bottom, a distance over which the displaced plastic loses the direct intimate bond with the non-displaced plastic due to the length of this distance, thus making the said displaced plastic less capable of contributing to the pull-out force. The term "pull-out force" refers to the force necessary for tearing out the screwed-in screw. However, a large free space for receiving displaced plastic results in a correspondingly small thread-bottom diameter (core diameter), and a consequence of this may be that, when such a screw is screwed in, in particular when it is screwed in over a relatively large number of thread flights, it is overloaded by the torque acting on it and breaks off.
The idea of displacing the plastic is also of primary importance in the design of the screw according to European patent specification 0,589,399. This screw, which is designed expressly for a high bearing depth of the thread flanks, is to achieve a material displacement such that the material is compacted on the thread flanks, specifically on those flanks which, as so-called load flanks, have to absorb tear-out forces acting on the screw. In order to influence the flow of material in this way, the screw possesses a conical thread bottom, the smaller core diameter of which is adjacent to the said load flank. The flow of the plastic is thereby to be directed towards the load flank, where the plastic is then to be compacted. This intended flow of plastic presupposes that the material is sufficiently heated and converted into flow and solidification, while the screw is being screwed in. The design of the known screw is therefore based on considerably softening, shifting and compacting the plastic while the screw is being screwed in, as a result of which the plastic is completely released from its original bond and its structure is changed. As a consequence of this, the load flanks of the screws have to be supported on this material which has been changed in such a way, but precisely this leads to a reduction in the tear-out forces of a screw thus screwed in.
The present invention departs from the idea, prevalent hitherto in the prior art, of the thread teeth penetrating as deep as possible into the plastic, which, at first sight, seems plausible in order to achieve high tear-out forces. For this purpose, according to a first variant, the following dimensioning may be used:
1. the outside diameter Do and the core diameter Dk of the thread form a quotient Q1=Do/Dk of a magnitude of 1.2 to 1.4; PA0 2. the axial spacing P of adjacent thread teeth forms, with the height H of the thread teeth, a quotient Q2=P/H which is between 2.75 and 2.9; PA0 3. the vertex angle of the thread teeth is about 30.degree.. PA0 1. the outside diameter Do and the core diameter Dk of the thread form a quotient Q1=Do/Dk of a magnitude of 1.25 to 1.65; PA0 2. the axial spacing P of adjacent thread teeth forms, with the height H of the thread teeth, a quotient Q2=P/H which is between 2.35 and 2.7; PA0 3. the vertex angle of the thread teeth is about 30.degree..
This dimensioning leads to the screw having relatively low thread teeth, so that the screw sits in the plastic with a relatively low depth of penetration. The advantage of this is that the structure of the displaced plastic is not impaired to any considerable extent. In order to achieve high tear-out forces in spite of the relatively low depth of penetration, the thread is produced with a relatively low pitch, so that a large number of thread flights engaged with the plastic are provided.
According to a second variant, the following dimensioning may also be used:
This latter dimensioning has, in addition, the following effect: the core diameter possesses a dimension which makes it possible to take into account the forming behaviour of a respective screw blank during its production by rolling, specifically in that the height of the thread teeth, which is building up on the core diameter, is still sufficiently large to displace the material of the screw in an advantageous way, in the region between the thread teeth, during rolling by the pressure exerted on the blank and, consequently, to produce accurately shaped thread teeth which, on the one hand, also have the advantage that they do not impair the plastic when they penetrate into it, but still have a height such that they can be shaped with considerable accuracy during rolling.
So that these effects, made possible by the dimensionings explained above, can be fully utilized, the screw according to the invention possesses a vertex angle of the thread which is about 30.degree..
A vertex angle of this kind for screws to be screwed into plastic is known per se, as shown, for example, by German patent specification 27 54 870. The aim, with this screw, is to make a relatively large free space between the thread flights available for the plastic into which the screw is screwed. The tendency followed by this screw amounts, therefore, to displacing as much plastic as possible by means of the thread teeth, that is to say, to working with a high depth of penetration, in order thereby to achieve high tear-out forces. The teachings of this patent specification do not therefore suit the prior art defined by German patent specification 39 26 000, since this prior art, apart from its particular suitability for movement threads, when it is applied to plastic screws, amounts to displacing as little plastic as possible with these screws.
The combination of the dimensioning features relating to the height and spacing of the thread teeth with the use of a particularly small vertex angle known per se results in a reinforcement of the principle of displacing as little plastic as possible when the screw is screwed into plastic, since the plastic mass to be displaced on account of the small vertex angle of 30.degree. is substantially smaller than in the case of a vertex angle of 40.degree., as emerges from German patent specification 39 26 000, the result of this being that the interspace existing between the thread flights, this being defined by the pitch of the thread, can be kept shorter, the consequence of this being that the number of thread flights to be anchored in a plastic part is increased correspondingly, if the screw-in length remains the same. This then leads to correspondingly increased tear-out forces.
High tear-out forces necessitate a corresponding lcad-bearing capacity of the screws both in the axial and in the tangential direction. The thinner the cross section of the screw relative to its outside diameter, the greater is the load on the screw, in relation to its cross section, both when it is being screwed in and when it is being tightened. So that, in this case, high tear-out forces car be withstood, it has hitherto been necessary for the screw material used for the known screws to have very high strength, the result of this being that highly tempered materials have had to be used for the screws in question here, these materials, on the one hand, being costly and, on the other hand, reacting sensitively to embrittlement occurring due to hydrogen diffusion. The consequence of this known effect may be that, in the known firmly tightened screws made of highly tempered material, time-delayed brittle fractures occur after they have been screwed in and tightened, for example by their heads splitting off after a few days. The screw dimensioning according to the invention provides screws having a relatively large cross section in relation to their outside diameter (as, for example, a look at FIG. 1 clearly shows). The screws according to the invention, by virtue of their design over their cross section, can therefore absorb considerably higher torques and axial forces than has been possible hitherto. This affords the possibility of producing the screw according to the invention from a material which has lower strength, as compared with conventional screws, that is to say materials can be used which, because of lower tempering, do not have the tendency to absorb hydrogen by diffusion and are therefore inclined to brittle fractures.